It is related to a device and method for writing data. Phase-change optical discs enabling repetitive recording, such as a DVD−RW and a DVD+RW, have become widespread over these years. The same data may be repeatedly written at the same position on a phase-change optical disc. However, when different data is subsequently written to the disc at the same data, the different data may not be correctly recorded on the disc. Accordingly, such defective recording must be reduced in phase-change optical discs.
Digital data is conventionally written to a phase-change optical disc by irradiating recording layers of the disc that are in a crystalline state with an intense laser beam. Then, the recording layers that have reached a dissolution temperature are rapidly cooled and changed to an amorphous state. An amorphous recording layer is normally referred to as a “mark”, and a recording layer between marks is normally referred to as a “space”, which is crystalline. Digital data is reproduced based on differences in reflectivity (light refraction index) between the crystalline portions and amorphous portions of the disc.
The writing of data to such a phase-change optical disc will now be described with reference to FIG. 1 using a DVD−RW as an example.
To write data onto a DVD−RW with a data writing device 1, the data that is to be recorded is transmitted from a host interface HOST I/F and stored in a buffer memory 5 via a controller unit 4, which is controlled by a CPU 3. A modulation circuit 50 incorporated in the controller unit 4 reads the data that is to be recorded from the buffer memory 5. The modulation circuit 50 modulates the data and provides the modulated data to a write channel unit 6 as write data WD. The modulation circuit 50 also provides a write clock signal WCLK to the write channel unit 6. In accordance with the write clock signal WCLK, the write channel unit 6 writes the write data WD to the DVD−RW with a pickup 7.
To enable high-density recording on the DVD−RW, the modulation circuit 50 in this type of data writing device 1 type modulates the data that is to be recorded by performing through eight-to-sixteen modulation in which eight bits of data are modulated to sixteen bits of data. The modulation circuit 50 further carries out a non-return to zero inverted (NRZI) technique to convert the data, which has undergone the eight-to-sixteen modulation, into modulation data MD in the NRZI format. The NRZI technique inverts the value preceding a data bit value of 1 and holds the value preceding a data bit value of 0. The modulation data MD in the NRZI format includes a mark and a space, each having a data length of 3T to 11T (where T is the length of a reference clock).
To enable synchronized detection during demodulation, the modulation circuit 50 adds two bytes of synchronization information SYNC to every ninety-one bytes of modulation data MD when generate the write data WD, as shown in FIG. 2. More specifically, the write data WD includes a plurality of synchronization frames, each of which is includes two head bytes of synchronization information SYNC and ninety-one bytes of modulation data MD. The frame (sector) structure of the write data corresponds to the physical sector of the DVD−RW.
As examples of the 2-byte synchronization information SYNC, FIG. 3 shows a plurality of (eight, for example) different types of synchronization signals SY0 to SY7 that identify frame numbers. For example, the synchronization signal SY0 is added to the head of the synchronization frame 1, and the synchronization signal SY1 is added to the head of the synchronization frame 2. Each of the synchronization signals SY0 to SY7 is either one of two synchronization signals, that is, a primary synchronization signal SYNC1 or a secondary synchronization signal SYNC2.
The primary synchronization signal SYNC1 and the secondary synchronization signal SYNC2 will now be described with reference to FIGS. 4A and 4B using the synchronization signal SY0 as an example. Each of the primary synchronization signal SYNC1 and the secondary synchronization signal SYNC2 is synchronization data in the NRZI format, and includes a mark or a space having a data length of 14T (a pattern in which fourteen consecutive clocks having the same level follow the fourteenth bit that has the value of “1”). In detail, the primary synchronization signal SYNC1 (FIG. 4A) includes an odd number of the value of “1” before the 14T appears. The secondary synchronization signal SYNC2 (FIG. 4B) includes an even number of the value of “1” before the 14T appears. When the 14T data of the primary synchronization signal SYNC1 is a space, the 14T data of the secondary synchronization signal SYNC2 is a mark. When the 14T data of the primary synchronization signal SYNC1 is a mark, the 14T data of the secondary synchronization signal SYNC2 is a space.
The modulation circuit 50 adds to the head of each frame as the synchronization information SYNC the one of the primary synchronization signal SYNC1 and the secondary synchronization signal SYNC2 with an absolute value of an accumulated digital sum value (DSV) that is closer to “0”. The DSV (digital sum value) is the accumulated value of points given in correspondence with the number of bits included in each bit string. Here, a point of +1 is given to each bit in a first level state (a bit value of “1” in this example) and a point of −1 is given to each bit in a second level state (a bit value of “0” in this example) as shown in FIGS. 4A and 4B. For example, when the accumulated DSV of the data preceding the synchronization information SYNC (synchronization signal SY0) added to the modulation data is 0 and a bit value of the modulated data preceding the synchronization information SYNC is 0, if the primary synchronization signal SYNC1 in FIG. 4A is selected as the synchronization information SYNC, the accumulated DSV will be −10 (0−10). If the secondary synchronization signal SYNC2 in FIG. 4B is selected, the accumulated DSV will be 4 (0+4). In this case, the secondary synchronization signal SYNC2 of the synchronization signal SY0 is selected as the synchronization information SYNC and added to the head of the synchronization frame (for example, the synchronization frame 1).
By lowering the absolute value of the DSV, low-frequency components of the write data WD can be reduced. This simplifies a binary circuit arranged in a reproducing device and reduces jitter that would be generated by a binary error.
However, when, for example, the same data is written repeatedly with the data writing device 1, the same data is repeatedly written at the same position. When the same data is repeatedly written at the same position on a phase-change optical disc, a mark and its vicinity would deteriorate due to thermal damage or the like. Thus, when different data (e.g., a space) is recorded next on the phase-change optical disc, the write data may not be correctly written. This decreases the rewritable number of times of the phase-change optical disc.
To prevent such deterioration of a phase-change optical disc, Japanese Examined Patent Publication No. 8-10489 describes a method for randomly changing the position at which the writing of data starts. In detail, the recording start position of the write data is changed randomly within a sector (frame) so that the same data is not written at the same position. This prevents deterioration of the phase-change optical disc.
In the DVD−RW standard and DVD+RW standard, the recording start position of the write data is provided with a small margin. Thus, the recording start position of the write data is changeable only within a limited range. Even if the recording start position of the write data is randomly changed, a portion of the 14T data of the synchronization information SYNC is always written at the same position as shown in FIG. 5. Accordingly, the position at which the same data portion is repeatedly written and its vicinity would deteriorate due to thermal damaged or the like. This decreases the rewritable number of times of the phase-change optical disc.